Free piston motor compressor



Nov. 26, 1963 J. G. H. OLLlER ET'AL 3,112,060

- FREE PISTON MOTOR COMPRESSOR Filed Jan. 22, 1962 a Sheets-Sheet 1 Nov. 26, 1963 J. G. OLLIER ETAL 3,112,060

FREE PISTON MOTOR COMPRESSOR Filed Jan. 22, 1962 3 Sheets-Sheet 2 v oE 9: U t U E. U .3 H 1 Jw m w, ,r hfi I 2 .v #5: l h v v :H .8? .5 5 2 $9 N: F 2i n 852 339 m: H, vs a. F 8.

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Nov. 26, 1963 Filed Jan. 22, 1962 141 FIG. 7

FREE PISTON MOTOR COMPRESSOR 5 Sheets-Sheet 5 FIG.1O

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United States Patent Ofiiice 3,112,060 Patented Nov. 26, 1963 3,112,060 FREE PISTON MOTOR COMPRESSOR Jacques Gaspard Honors: flllier, 17 Rue (in General Henrion Bertier, Neuilly sur-Seine, France, Georges Frtlric Grosshans, 53 Rue de Boulainvilliers, Paris,

France, Raymond Antoine Parre, Sceaux, Kurt Hellweg, Antony, and Guillaume Cariou, Paris, France;

said Parre and said Cariou assignors to S. N. Marep,

Paris, France, a national corporation of France, and

said Hellweg assignor to Societe Civile pour lEtude dEngins a Pistons Libres EARL, Bouiogne, France,

a national corporation of France Filed Jan. 22, 1962, Ser. No. 167,842 Claims priority, application France Feb. 6, 1959 6 Claims. (Cl. 230-56) This application is a continuation-in-part of application filed February 5, 1960, Serial Number 6,939, now abandoned.

This invention relates to motor-compressors with free pistons, of the type wherein the motor comprises two opposed pistons which compress between them air into which is injected fuel, the ignition being effected by compression, and the compressors comprise two cylinders one at each end of the motor cylinder with a compressor piston in each compressor cylinder.

The invention is concerned particularly with motor compressors intended to draw in a gas under supply pressure and to deliver it at a higher pressure, as is the case, for example, with motor compressors for the conveyance of natural gas.

In free-piston motor compressors of the prior art, the stroke of the motor-pistons is determined by the amount of load the motor pistons are subjected to during their displacement, and the ignition is effected in accordance with a predetermined position of the motor pistons on their return stroke. Any variation of said load, and more particularly any variations of the oounterforces urging the motor pistons towards said predetermined position thereof will unbalance the operation of the motor.

In free-piston motor compressors of the type described, the delivery pressure built up by each compressor piston reacts on the compressor piston on completion of its compression stroke thereby causing the compressor piston to transmit work to its associate motor piston during the return stroke thereof, such transmission of work tending to move the motor pistons beyond said predetermined firing position thereof so as to unbalance the operation of the motor.

It is the main object of the invention, therefore, to render the compressor pistons more stable in operation and independent of variations in load.

It is another object of the invention to provide a freepiston motor compressor of the type described, in which the total work done by the compressor piston on the associate motor piston amounts to zero or substantially thereto.

It is another object of the invention to control the operation of each compressor, unit of a conventional freepiston motor compressor during the inward stroke of its piston so that the total work done by such piston on its associate motor piston during a complete return or inward stroke amounts to zero or approximately thereto, and this moreover for variable intake pressures and stroke length.

It is another object of the invention to provide a freepiston motor compressor intended to deliver and store gas under very high pressures without having the motor pistons driven beyond a predetermined firing position under the effect of said very high gas pressures.

Although the novel features which are characteristic of this invention are pointed out more particularly in the claims, the nature of the invention will be better under-- stood by reference to the following description, taken in connection with the accompanying drawings in which two specific embodiments have been set forth for purposes of illustration.

In the drawings:

FIGURE 1 is a diagrammatic axial section of an improved free-piston motor compressor for gas;

FIGURES 2 and 3 are diagrams illustrating the operation of the compressor;

FIGURE 4 is a diagrammatic axial section of an improved free-piston motor compressor for compressing gas to very high pressures;

FIGURES S to 8 are part axial sections illustrating one of the compressor pistons of FIG. 4 in various positions during its return stroke;

FIG. 9 is a diagram showing the pressure acting on the opposite sides of the compressor piston of FIGS. 5 to 8, during the return stroke, with the pressure being plotted in ordinates and length of piston stroke in abscissae; and

FIG. 10 is a diagram generally similar to FIG. 9 but wherein the ordinates represent the forces applied to the piston surfaces rather than the pressures.

Referring to FIGURE 1, a motor cylinder 1 is surrounded by a gas-tight casing 2 and is provided with scavenging ports .3 and exhaust ports 4, the latter being connected to an exhaust passage 4a. The pistons 6 and 6' of the scavenging pump are secured to the two motor pistons 5 and 5' and the compressor pistons 7 and 7' are secured to the former by means of rods 8, 8. The two groups of pistons are synchronized by an appropriate kinematic system comprising, for example, two racks 9 and 9 fast with the pistons 6 and 6' respectively and interengaged by a pinion 10 mounted in known suitable manner in the casing 2.

The cylinders 11 and 111 of the compressors are surrounded by external casings 12 and 12 which are spaced outwardly and endwise from the cylinders 11 and 11' and which are secured in gas-tight manner on end cover plates 13 and 13' of scavenging cylinders 16 and 16'. The rods 8 and 8' of the compressor pistons pass through appropriate gas-tight stuffing boxes 14 and 14' in the plates 13 and 13' which are attached to flanges 15 and 15 integral with the outer ends of the scavenging-pump cylinders 16 and 16', the inner ends of which are connected to the casing 2, for example by bolted flanges 17 and 17'.

When the motor pistons 5 and 5 are near the end of their inward stroke, the fuel is injected in known manner, by an injector 18, into the air compressed between these pistons and the ignition is produced simply by the compression. By the effect of the pressure of the combustion gases, the pistons then execute their outward stroke. The scavenging air is drawn through valves 19- and 19' and the air enclosed in the cushion spaces 20, 20' beyond the pistons 6, 6' is compressed. The piston 5' uncovers the exhaust ports 4 and the piston 5 uncovers the scavenging ports 3.

During the return inward stroke, the scavenging air is forced into the inside of the casing 2 through the valves 21, 2:1 (in the direction of the arrows f). The work necessary for the return stroke is furnished by the expansion of the air in the cushion spaces 20 and 20'. In effect, and as will be seen hereinafter, the work performed during this period by the compressor pistons is reduced practically to zero due to the arrangement forming the subject of the invention. The motor pistons then close the scavenging ports 3 and thereafter the exhaust ports 4 and, starting from this point, the air enclosed in the motor cylinder 2 is again compressed to a pressure such that ignition and combustion of the fuel take place in a suitable manner when the pistons are at the end of their inward stroke. To ensure this compression equally as well for long strokes as for short strokes, it is essential to modify the initial pressure or the mean pressure in the cushion spaces 20, 20'. The initial pressure should be augmented when the strokes are shorter or, which amounts to the same thing, the mean pressure in the cushion spaces 29, 20* should remain substantially constant for all strokes.

By way of indication, a known system of controlling the mean pressure in the cushion spaces has been shown. A regulator 22 comprising two coaxial cylinders 22a, 22b, which are separated by a partition 220, of which one, 22b, is open to atmosphere at 22d, contains a slide valve 23 with three heads, the upper one in the cylinder 22a and the two others in the cylinder 22b. The lower face of the upper head is subject at 24 to the mean pressure of the cushion spaces 20, 20. A spring 25 holds the slide valve in equilibrium. A passage 26b connects the two cushion spaces to the side 24 of the upper head of the slide valve and a passage 27 connects the casing 2 of the motor cylinder 1 to the cylinder 22b of the regulator. When the stroke of the pistons increases, mean pressure in the spaces 20, 20' tends to increase, the slide valve is displaced upwardly and air in the said spaces is discharged to atmosphere by a passage 26a connecting the passage 26 to the open end 22d of the regulator until there is again equilibrium and the slide valve has returned to its initial position, closing the passage 26a. If on the other hand, the stroke of the pistons decreases, the mean pressure in the spaces 20, 20 tends equally to decrease, the slide valve descends and the scavenging air passes from the passage 27, through a non-return valve 28, to the passages 26a and 26 and supplies the said spaces 20, 20 until there is again equilibrium.

The construction so far described is known. The compressor part of the apparatus forming the subject of the invention will now be described. The description will be given by reference to the left-hand compressor (FIG- URE 1), but it is evident that the right-hand compressor is the same and functions in the same manner.

Each compressor comprises, as stated above, a casing 12, preferably of cylindrical section, fixed in gas-tight manner (for example welded) to the adjacent cover plate 13. This casing, which is closed at its outer end by a head or end wall 29, contains the cylinder 11 proper of the compressor. This latter is open at its inner end and closed at its outer end by a head or end wall 30 formed with a suction port fitted with a suction valve 31 and with a delivery port fitted with a delivery valve 32. The space between the enclosure 12 and the cylinder 11 is divided into three compartments 36, 37, and 38 by two radial partitions 33, 34 and by an axial partition 35. A

port 39 establishes communication between the compartment 36 which is a suction compartment, and the interior of the cylinder 11 on the side towards the open end of this cylinder, which also communicates freely with the compartment 37, which is an admission compartment while the cylinder space on the outer side of the piston 7 communicates by way of the suction valve 31 with the compartment 36 and by way of the delivery valve 32 with the compartment 38 which is a delivery compartment. The admission pipe 40 for the gas to be compressed, opens into the admission compartment 37 and the delivery pipe 41 for the compressed gas is connected with the compartment 38. The operation of the compressor is as follows: At the starting up of the motorcompressor there is introduced into the spaces 20 and 20, according to the known technique of diesel motor working, compressed air from an appropriate source such as a bottle for example. This air arrives through pipes 42 and 44 and past a valve 43, inserted in the pipe 26, under a pressure suificient for bringing the pistons 5, to the end of their inward stroke and produces ignition by compression of the combustible mixture imprisoned between the pistons in the cylinder 1.

In describing the operation of the compressors, it will be presumed, merely by way of example, that the gas pressure in the supply pipe 40 is about lbs. per square inch and that the delivery pressure in the pipe 41 is about lbs. per square inch. With the parts in the position shown in FIGURE 1, the operation may be described with reference only to the left hand set of pistons, because a similar operation occurs with the right hand set. The compressed air in the space 20 behind the piston 6 will drive the pistons 6, 5 and 7 to the right to perform the inward stroke. At the start of this stroke there is also a compression space behind the face 7b of the piston 7, as shown in FIGURE 1, and this space contains gas at the delivery pressure of 120 lbs. operating on the larger area of the face 7b as compared with pressure of 80 lbs. acting on the smaller area of the face 7a and the piston 7 will therefore do positive work on the pistons 6 and 5 whilst the pressure gas at 120 lbs. is expanding and reducing its pressure to 80 lbs. during the first portion of the inward stroke which is represented by a to b in the work diagram in FIGURE 3. In the next portion of the stroke, namely from b to c in FIGURE 3, pressure on the face 7b is kept up to 80 lbs., by the admission of gas past the inlet valve 31, this gas freely entering the chamber 36 and coming from the supply pipe 40, via the open end space of the cylinder 11 and the port 39 which is still uncovered by the piston 7. During this b-c period there is an equal pressure of 80 lbs. on the two faces 7a and 7b but the piston 7 will still do a small amount of positive work on the piston 6 and 5 due to the fact that the area of the face 7a is smaller than the area of the face 7b. At the point e in FIGURE 3, the piston 7 covers the port 39 as its face 7a moves past the inner end of that port but almost immediately (the length of the port 39 in this example being about equal to the length of the piston 7), the face 7b will move past the outer end of the port. This will put the chamber 36 in direct communication with the cylinder space behind the face 7c so that there will be an equalization of pressure in the said chamber and space. During the further progress of the inward stroke of the piston 7, the total effect of the pressure on the face 7b decreases from the point c, in FIGURE 3, to the point 0, at which latter point the said total effect becomes equal to the total effect of the admission pressure on the smaller area of the face 7a, so that the aforesaid positive work decreases to Zero value. Now, from the point 0 to the point d which indicates the end of the inward stroke of the piston 7, the pressure on the face 7b decreases rapidly to a value below that of the admission pressure. The work done by the piston 7 on the pistons 6 and 5 is now reversed from a positive to a negative value, due to the admission pressure of 80 lbs. acting on the face 7a opposing the reduced pressure of, say, about 40 lbs. acting on the face 7b. By judiciously selecting the diameter of the piston 7 and that of its rod 8, its is possible to equalize the positive and negative work area of the diagram. Thus, in FIGURE 3 it will be seen that the positive area a-b-c-a-g is substantially equal to the negative area o-h-d-o, for one admission pressure and that a similar result is obtained with a lower admission pressure when the said areas become a-b'c0'--g' and o-h-d'o respectively. It will also be seen from the diagram in FIGURE 2 that this substantial equality between the two areas is the same for either a long or a short stroke. For a long stroke, the pressure on the face 7b is indicated by abcdef and the pressure on the face 7a by g-h-e-f. For the short stroke, the pressure on the face 7b is indicated by i-cdek and that on the face 7a by lh-ek.

During the first portion of the outward compression stroke of the piston 7, the latter will displace some of the gas from the cylinder space through the port 39 into the chamber 36 and thus elevate the pressure in the latter. The piston 7 then closes the port 39 but almost immediately reopens it to give communication between the chamber 56 and the gas supply from the pipe 40 via the chamber 37. The gas displaced into the chamber 36 during the first portion of the stroke provides a cushion of gas sufficient for reducing the shock which otherwise would be produced when the face 7a uncovers the port 39 to admit the full supply pressure from the chamber 37 into the chamber 36.

The motor compressor shown in FIG. 4 is intended to compress gas to a very high delivery pressure, and comprises two motor pistons 101 and 1' connected by piston rods 102 and 102 with the scavenger pump pistons 103 and 103'. Both motor pistons i101 and 101' are slidingly mounted within a common cylinder casing 107, while the scavenger pump pistons 103 and 103' are positioned beyond the opposite ends of the motor cylinder casing 107 and slide within a pair of cylinder casings "117 and 117 coaxially surrounding and extending beyond the motor cylinder casing 107 as shown. Each scavenger piston 103, 103' is connected at its outer side by way of a Cardan joint of the like 105, 105, to a piston rod 104, 104', which projects outward through the related end of its cylinder 117, 117' and into a compressor cylinder casing 125, 125' secured to the outer I end of the scavenger pump casing. Each piston rod 104, 104 extends through the related wall of casing 117 or 117' by means of a gland 133, 133', and is connected at its outer end with a compressor piston 106, 106, respectively.

The common motor cylinder 7 is for-med with a scavenging port 108 an exhaust port 109 communicating with an exhaust manifold 110, and further ports 11 1, 111 which provide communication between a scavenge-r air reservoir 113 secured to an outer side of cylinder 107, and the outer chambers 1-12 or 112 thereof. In operation, there is thus provided in each of said outer cylinder chambers an air-cushion between the outer face 114-, 114 of the motor pistons 101 and 101' and the cylinder heads 115 and 115 through which the piston rods 2, 2' extend by Way of glands 116 and 1 16', respectively.

The scavenger pistons 103, 1033 slide in the cylinder casings 1:17, 117' which latter are provided with automatic intake valves 118, 118 and automatic discharge valves 119, 119.

Scavenging air is delivered from the outer chambers of cylinders 117, 117' into the scavenging air tank 113 through conduits 120 and 120'. The space defined between the inner surface 121, 121' of the scavenger pistons 103, 103', and the separating wall 122, 122' provides what may be termed a suction air cushion 123, 123', and is provided with automatic discharge valves 124, 124'.

Each compressor piston 106, 106 divides its cylinders 125, 125' into a compression chamber 141, 141 and an admission or intake chamber 134, 134. The compression chamber 141, 141' is connected with a delivery line 1 13, 143 through a discharge valve 127, 127' while the intake chamber 134, 134 is connected by a non-return valve 135, 135' with a supply line 14-2, 142' connected with the source of gas that is to be compressed.

An intermediate chamber 130, 130' connects the intake chamber 134, 134 with compression chamber 141, 141 through a port which in this embodiment comprises a pair of openings 123- 129, or 128129 formed in the wall of cylinder 125, 125, said openings being controlled by the pistons 106 and 106', and also through an automatic inlet valve 126, 126'. The effective axial distance between the openings 12S and 129 (or 128' and 129'), is substantially equal to the length of piston 106' (or 106'), and these openings are positioned relatively to the displacement stroke of the piston in a manner that will be later explained.

The reciprocations of the two assemblies just described are synchronized with each other by means of a rack and gear system including racks 136, 136' respectively secured to the scavenger pistons 103, 103' and meshing with a common pinion 137. The apparatus operates as follows.

As the motor pistons 10 1, 101' are positioned adjacent the inner neutral points of their stroke, as shown in FIG. 4, fuel is injected through a nozzle 138 and the combustion gases produced expand and propel'l the two separate reciprocatory assemblies outward. The face 114, 114' of the motor piston :101, 101 seals the port 111, 111 and air is compressed in the pressure chamber 112, 112. Scavenger piston 103, 103' compresses the scavenging air and discharges the same through check valves 119, 119' and conduit 120, into the scavenging air tank 113. The chamber 123, 123' increases in volume so that the air pressure in it drops to a subatmospheric value.

Towards the end the outward stroke of the assemblies the motor piston 101 uncovers the exhaust port 109, and permits discharge of the exhaust gases through exhaust manifold 110 in the direction of arrow 139' to a suitable receiver unit, suuch as a turbine (not shown). Thereafter, motor piston 101 uncovers the scavenger port 103 and the motor cylinder 107 is swept clean with compressed scavenging air also discharged through the exhaust port 109.

The motor pistons are then propelled inwardly partly by the action of the pressure cushions 1'12 and 1:12 which restore the energy stored therein during the outward stroke, and partly by the action of the suction in the compartments 123 and 123 which diminish in volume until the pressure therein is restored to atmospheric. Any overpressures due to leakage past the pistons are avoided by the opening of the automatic check valves 124, 124. The scavenger pump pistons draw in air through the check valves 118, 118.

During the motor cycle just described the compressors perform the following operating cycle. It is assumed that they serve to compress gas from an initial pressure of 21 kg./cm. to a final pressure of 6 1 kg./cm. During the outward stroke of each of the reciprocatory assemblies, the associated compress-or piston, e.g. 106 first closes the port 129 and then the port 128. After port 123 has been closed the body of gas entrapped in the cylinder is compressed and discharged through check valve 127. The intake chamber 134 increases in volume and when the pressure therein, which at the initial point of the outward stroke was higher than the supply pressure in line 142 as will appear later, has dropped to a value equal to said input gas supply pressure, the input gas raises the non-return valve 135 and enters chamber 134.

Each of the reciprocating assemblies is now positioned as shown in FIG. 5. The compressed gas, compressed to the final outlet pressure of 61 k'g./cm. has been discharged through the outlet valve -'127 and the residual gas in chamber 141 is at the same pressure value. The compressor piston 106 is now at the end of its outward stroke, and the corresponding operating point on the pressuredisplacement diagram shown in FIG. 9 is at the point a. As the piston now recedes, the gas entrapped in chamber 14-1 expands, while the gas entrapped in the space comprising chamber 134 and chamber is compressed. The said space is sealed since the valve 126 is now closed by the relatively high pressure which is still prevailing in compression chamber 141. This corresponds to the curve section ab on the curve of FIG. 9. Valve is held closed by the pressure in chamber 134 being higher than the supply pressure.

After the decreasing pressure in compression chamber 141 and the increasing pressure in chamber 134 have reached a common level, as indicated at point b of the diagram FIG. 9 and which level is 24 kg./cm. in the example under discussion, the piston is positioned as shown in FIG. 6, and the corresponding operating point on the diagram of FIG. 9 is b.

Thereafter, as the piston continues to recede towards the position shown in FIG. 7, wherein piston 106 has just closed port 129 and hence cut off the communication between chamber 134 and chamber 136 (as indicated by the position on the diagram), the pressure on both faces of the piston remains substantially constant, dropping slightly from 24 to 23.5 kg./cm. owing to the difference in the volumes generated by the unequal surface areas on the two sides of the piston.

In practice the pressure in chambers 134 and 130 on the one side and 141 on the other may differ slightly during the displacement represented by section b-c of the curve, owing to the resistance of check valve 126.

Ports 128 and 129 are so positioned that, as piston 106 is about to close port 129 on its return stroke, the pressure in compression chamber 141 has dropped to the value of 23.8 kg./cm. and hence, at the time the piston will again pass through the same position during its outward or compression stroke, the port 129 will be uncovered at a time when the pressure in the compression chamber 141 is at the same value, so that the gases will flow easily and smoothly from the supply to the compression chamber.

From the point e of the diagram the body of gas entrapped in compression chamber 141 expands continually down to a pressure value of 14.8 kg./cm. at the end of its inward stroke (FIG. 8), corresponding to point d of the diagram.

During the return stroke, the pressure applied to the larger piston surface has therefore been changing in accordance with the broken line abc-d of the diagram of FIG. 9.

During the same period, starting at the instant the piston has covered inlet port 129 (point 0 of the diagram of FIG. 9, the gas entrapped in chamber 134 is gradually compressed up to a pressure of 57.9 kg/cm. at the end of the return stroke of the piston (FIG. 8) holding nonreturn valve 135 closed. This is expressed on the diagram by the rapidly rising curve section cn, hereinafter called the compensating pressure.

Throughout the entire return stroke, the smaller piston surface was therefore exposed to a varying pressure as indicated by broken line lb-cn (FIG. 9).

The pressure applied to the larger area of compressor piston 106, during the return stroke of said piston, performs an amount of work which will be called positive while the pressure applied to the smaller annular area will be regarded as performing negative work. The work performed is represented by the integral of the pressure forces with respect to displacement, i.e. the areas under the curves.

In order that the total work supplied by the compressor piston to the motor piston throughout the return stroke shall be substantially zero, with due allowance to the relative values of the areas of the opposite faces of the piston, the afore-mentioned positive and negative works must be equal.

In FIG. 10 which is generally similar to the diagram of FIG. 9, the ordinates represent the forces applied to the opposite piston faces by the corresponding pressures, with due allowance for the difierence in areas of said surfaces. In order that the respective amounts of Work (or integrals of force over displacement) indicated above shall be equal in absolute value, it is necessary that the two areas ab-cdef and l-v-u-n-e-f-l be equal, or equivalently, that the two cross-hatched areas abcp-uvla and pndp be equal. It will be readily understood that by suitably dimensioning the compressor, the presence of the non'return valve 135 makes it possible to increase the compensation pressure in intake chamber 134 to a considerable value in spite of the relatively low value of the supply pressure, in order to compensate for the work performed by the expansion of gases in the compression chamber from the relatively high pressure value at which the outlet gases are discharged from the compressor.

During the compression stroke, the pressure in compression chamber 141 is initially 14.8 kg./cm. (point d of the diagram in FIG. 9) i.e. less than the supply pressure. It gradually rises to a final value of 23.8 kg./cm. as the piston uncovers port 129 (point e of the diagram), then the pressure follows curve segment c-g and finally reaches the output pressure value 61 kg./cm. Check valve 127 then opens and the compressed gas is discharged during the last stage of the piston stroke indicated by curve segment g-a.

Moreover, during the return stroke the motor piston first seals scavenger port 108 and then exhaust port 109. From this point the compression in motor cylinder 107 commences. This compression is therefore solely produced by the action of the pressure in chambers 112, 112 on the pistons 101, 101' and of the suction in chambers 123, 123 on the pistons 103, 103, regardless of the intake pressure of the compressor, so that the operation of the motor, especially as to the conditions during the return stroke, is not affected by the presence of the compressor.

A motor compressor constructed and operating in accordance with the invention need not be stopped completely when the demand for compressed gas is temporarily suspended and re-started when the demand is resumed, but may continue running idly during the temporary suspension of demand.

It is to be understood that the constructions and arrangements hereinbefore described and illustrated by way of example, are capable of modification without departing from the invention as hereinafter defined by the claims. For instance, although it is advantageous for the port 39, 39' to have the form of elongated slots, it is possible to achieve the object of the invention with other port formations.

What is claimed is:

1. in a motor compressor having a pair of free motor pistons and two compressor units, each unit comprising a cylinder one end of which is open and the other closed, a compressor piston slidable in said cylinder and connected respectively to one of said motor pistons, valved inlet and outlet ports formed in said closed cylinder end, means providing a chamber externally of said cylinder, means for conducting gas under supply pressure to said open cylinder end, and an opening in the wall of said cylinder, said chamber communicating with said inlet port and said opening to provide a piston-controlled gas passage therebetween which is closed by the compressor piston at such time before the completion of the return stroke thereof that the total work done by the compressor piston on the associate motor piston during a complete return stroke amounts to zero or substantially thereto.

2. A motor-compressor according to claim 1, in which said opening has the form of an axially extending slot the length of which is substantially equal to the length of the compressor piston.

3. A motor compressor system comprising a motor cylinder; a pair of free motor pistons reciprocable therein; a pair of compressor cylinders positioned beyond the ends of the motor cylinder; compressor pistons reciprocable in said compressor cylinders, means mechanically coupling each motor piston to a related compressor piston; combustion means in the motor cylinder for reciproeating said motor pistons and compressor piston; each compressor piston dividing its cylinder into an intake chamber and a delivery chamber; means connecting said intake chambers to a source of gas to be compressed and means connecting said delivery chambers to a compressed gas delivery; means providing a communication from said intake to said delivery chamber during a portion of the stroke of said compressor pistons to deliver compressed gas to said delivery; and check valve means interposed etween said source and each intake chamber and positioned to oppose the flow of said gas from said intake chambers to said source whereby to develop a compensatory pressure tending to equalize the amounts of work performed by the gas pressure forces on the opposite sides of each compressor piston during the reciprocation thereof.

4. A motor-compressor system comprising a motor cylinder; a pair of free pistons reciprocable therein; a pair of compressor cylinders generally aligned with the motor cylinder; compressor pistons reciprocable in said compressor cylinders; means coupling each motor piston with the related compressor piston; combustion means in the motor cylinder for reciprocating said pistons; each compressor piston dividing its cylinder into an intake chamber connected with a source of gas to be compressed and a delivery chamber connected to a pressure-gas delivery line; port means in each compressor cylinder wall adapted to be covered and uncovered by the compressor piston during reciprocation thereof; a passage connecting said port means with the end of the related compressor cylinder to provide communication from said intake to said delivery chamber thereof so as to deliver compressed gas to said delivery; a check valve opposing flow from said delivery chamber into said passage; and a check valve opposing flow from said intake chamber to said source; whereby a compensatory pressure will be developed in said intake chambers during the return stroke of said pistons tending to equalize the integral of the pressure forces acting on both sides of each compressor piston during reciprocation thereof.

5. A motor-compressor system as claimed in claim 4 wherein said port means comprises at least a pair of axially-spaced ports in said compressor cylinder wall.

6. A motor-compressor system as claimed in claim 4 wherein said port means comprises at least one axially elongated slot in said compressor cylinder wall.

Pescara Oct. 22, 1929 Dodge Dec. 13, 1932 

1. IN A MOTOR COMPRESSOR HAVING A PAIR OF FREE MOTOR PISTONS AND TWO COMPRESSOR UNITS, EACH UNIT COMPRISING A CYLINDER ONE END OF WHICH IS OPEN AND THE OTHER CLOSED, A COMPRESSOR PISTON SLIDABLE IN SAID CYLINDER AND CONNECTED RESPECTIVELY TO ONE OF SAID MOTOR PISTONS, VALVED INLET AND OUTLET PORTS FORMED IN SAID CLOSED CYLINDER END, MEANS PROVIDING A CHAMBER EXTERNALLY OF SAID CYLINDER, MEANS FOR CONDUCTING GAS UNDER SUPPLY PRESSURE TO SAID OPEN CYLINDER END, AND AN OPENING IN THE WALL OF SAID CYLINDER, SAID CHAMBER COMMUNICATING WITH SAID INLET PORT AND SAID OPENING TO PROVIDE A PISTON-CONTROLLED GAS PASSAGE THEREBETWEEN WHICH IS CLOSED BY THE COMPRESSOR PISTON AT SUCH TIME BEFORE THE COMPLETION OF THE RETURN STROKE THEREOF THAT THE TOTAL WORK DONE BY THE COMPRESSOR PISTON ON THE ASSOCIATE MOTOR PISTON DURING A COMPLETE RETURN STROKE AMOUNTS TO ZERO OR SUBSTANTIALLY THERETO. 